1. Field of the Invention
The present invention relates to a dielectric filter, a composite dielectric filter, and a communication apparatus including the same, for use in a high-frequency band.
2. Related Art
FIG. 8 shows a structure of a dielectric filter using a dielectric block, which is mainly used in the micro-wave band. In FIG. 8, reference numeral 1 indicates a dielectric block of a rectangular parallelepiped configuration. Inside of the dielectric block 1, resonance-line holes indicated by 2a, 2b and 2c, are disposed, and inside the inner surface of each hole, an inner conductor is disposed to form a resonance line. On an outer surface of the dielectric block 1, a ground electrode 3 is disposed, and terminal electrodes 6 and 7 are disposed such that they are insulated from the ground electrode 3.
In FIG. 8, the resonance lines respectively disposed on the inner surfaces of the resonance-line holes 2a, 2b, and 2c make comb-line couplings in sequence, and the terminal electrodes 6 and 7 make a capacitive coupling to the resonance lines formed on the inner surfaces of the resonance-line holes 2a and 2c. In this way, a dielectric filter having a band pass characteristics of three-stage resonators is provided.
In a prior art dielectric filter shown in FIG. 8, however, it is difficult to make the coupling between specified adjacent resonance lines of the aligned plural resonance lines different from the coupling between the other resonance lines. For example, the strength of coupling cannot be changed by changing the position (the step position) where the inner diameter of a resonance-line hole 2 is changed. In a sectional view shown in FIG. 9A, for example, when the depth Lo of a step position (the depth from the end face of the dielectric block on the open-end side) is increased while maintaining a uniform pitch Po for aligning the resonance-line holes 2a, 2b, and 2c. the capacitive coupling between the adjacent resonance lines is strengthened. Additionally, if the pitch mentioned above is kept uniform and the only change is to make the step positions of the resonance-line holes 2b and 2c lower, the capacitive coupling between the resonance lines of the resonance-line holes 2a and 2b is strengthened. As a result, for example, as shown in FIG. 9B, in order to strengthen the coupling between the resonance lines formed in the resonance-line holes 2b and 2c without changing the coupling between the resonance lines formed in the resonance-line holes 2a and 2b, it is necessary to make a design for lowering down the step position L1 and making the pitch P1 between the resonance-line holes 2a-2b wider than Po. Consequently, the dielectric block and the dielectric filter overall are thereby made larger in size.
To overcome the above described problems, preferred embodiments of the present invention provide a dielectric filter, a composite dielectric filter, and a communication apparatus including the same, in which the coupling between specified resonance lines among the adjacent resonance lines can be independently determined without changing a pitch for aligning the resonance-line holes.
One preferred embodiment of the present invention provides dielectric filter comprising: a dielectric block; a plurality of resonance-line holes aligned therein; a resonance line disposed on an inner surface of each of the resonance-line holes; and an outer conductor disposed on an outer surface of the dielectric block; wherein one end of the resonance-line hole is a short-circuited end; a sectional area of at least one of the resonance-line holes is changed at a predetermined portion; the predetermined portion of at least one of the resonance-line holes is set to be different from that of the other of the resonance-line holes along to the axial direction of the resonance-line hole and at a side opposed to the adjacent resonance-line hole.
According to the above described arrangement and structure, the coupling between a specified resonance line and the adjacent resonance line on the left side of the specified resonance line can be strengthened, while the coupling between the specified resonance line and the adjacent resonance line on the right side of the specified resonance line can be weakened, without changing the pitch for aligning the resonance-line holes, for example.
Accordingly, the pitch for aligning the resonance-line holes can set to be uniform, so that a molding metal die for making a resonance-line hole with respect to the dielectric block can also be used commonly in the case of formation of a dielectric filter having a different characteristic. Further, without increasing the size of the dielectric block, a compact dielectric filter having a desired characteristic can be obtained.
In the above described dielectric filter, the surface of the part where the sectional area of the resonance-line hole is changed may be set to be inclined to the axis direction of the resonance-line hole. This structure permits the molding metal die of the dielectric block to be easily produced. In addition, when a resonance-line hole is formed in the dielectric block by cutting processing, the production can be easily conducted.
Furthermore, in another preferred embodiments, three or more signal input-output terminals are disposed on a dielectric block for performing input and output of signals by coupling to specified resonance lines of a plurality of resonance lines so as to form, for example, a composite dielectric filter such as a duplexer, a diplexer, or a multiplexer.
Furthermore, the dielectric filter or the composite dielectric filter according to the present invention is used as at least one of a transmission filter and a reception filter so as to form an antenna duplexer having a transmission signal input unit, a reception signal output unit, and an antenna connection unit.
Furthermore, the dielectric filter or the composite dielectric filter according to the present invention is disposed in a high-frequency circuit section. This arrangement permits a compact and lightweight communication apparatus to be obtained.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like reference labels denote like elements and parts and redundant description of reference labels is therefore unnecessary.